A number of commercially useful reactions are exothermic in nature and generate large amounts of heat which needs to be removed. On an industrial scale, for many reactions, it is possible to remove the heat of reaction in the form of a circulating fluid, such as water, which is raised for steam, which can then be used for another purpose, for example, for generating power.
An example of such a reaction is the Fischer Tropsch (FT) reaction which converts synthesis gas (syngas) to linear hydrocarbons. The heat of reaction may be removed by partially vaporising boiler feed water (BFW) which is introduced into thermal contact with and receives heat given off by the exothermic FT reaction vessel, tubes, or channels wherein the FT catalyst is contained and the reaction with syngas takes place. Temperature is one of the most critical operating parameters of the FT reaction, controlling the carbon monoxide conversion present in the syngas (per pass CO conversion), in addition to the length of hydrocarbon chains synthesized (selectivity).
The temperature of the coolant is selected to provide the desired cooling capacity for the reaction. In the case where vaporisation of water is used to cool the reaction, the temperature of the coolant is controlled by the pressure at which the steam is generated. Coolant which is partially vaporised as a consequence of the exothermic reaction is passed from a reactor to a reservoir, essentially a steam drum, where the vapour and liquid are separated. The vapour (steam) may be further used for heating or power generation. The liquid may be used further in the process or treated and/or recirculated as a coolant in the process. The pressure at which the steam drum is operated dictates the saturation temperature of the liquid, which is then recycled back into the reactor as coolant.
In an industrial process, there is always a drive to maximize the production capacity of a reaction train in order to exploit economics of scale to minimize unit cost of production. The practical limit of production capacity of a single reaction train may be driven by the maximum size of major equipment or other factors. Thus, in order to meet the overall desired production capacity of a facility, multiple reaction trains may be required. When multiple reaction trains are used, it is commonly desired to optimize the output and maximize the ease of operation of each reaction train used with the associated duplication of equipment for independent operation.
In the case of a conventional exothermic catalytic process, in particular, where the activity of the catalyst declines over time, each reaction train is designed to operate independently at maximum production capacity, with an operating strategy for different reactor temperatures in different trains as the reaction catalyst activity declines over time and is compensated for by increasing the reaction temperature. In this case, the coolant temperature profiles may be different between multiple trains in order to optimize production. In the case of a process cooled by vaporisation of water, such as a Fischer-Tropsch (FT) process, reaction temperature is controlled by the pressure of the resulting steam, which is typically sent to a separate vessel (“steam drum”) in which the pressure of the steam is controlled. In this case, the temperature and pressure of the steam drum may be different at any point in time for each reaction train. To date, the FT process has been designed so as to utilise a single steam drum per reaction train. This means that the reaction temperature of each reaction train can be controlled by the operating pressure of the associated single steam drum. A similar approach has been adopted in other exothermic industrial processes where the heat of reaction can be controlled by a recirculating coolant, such as the generation of steam from water.